Recently, with the coming of an advanced information society, development of optical materials such as an optical fiber and an optical waveguide in the field of optical communications and optical materials such as a liquid crystal orientation film and a color-filter protective film in the field of display devices has been advanced. Particularly in the field of display devices, a light and excellently flexible plastic substrate has been studied as an alternative for a glass substrate, and development of a display which can be bent and rolled has been aggressively made. In the circumstances, a higher performance optical material that can be used for such purposes has been demanded.
Polyimides are essentially colored in yellowish brown by intramolecular conjugation and charge-transfer complex formation. As a means to reduce the coloration, for example, a method of expressing transparency by inhibiting formation of intramolecular conjugation and charge-transfer complex by introducing fluorine, providing flexibility to the main chain, or introducing a bulky side chain is proposed. In addition, methods of expressing transparency by using semi-alicyclic or wholly alicyclic polyimide resins that do not, in principle, form charge-transfer complexes are proposed.
Patent Document 1 discloses that a thin-film transistor substrate is obtained by forming a thin-film transistor on a transparent-film substrate of a polyimide having an aliphatic group as a tetracarboxylic acid component residue by use of a conventional film-forming process in order to obtain a thin, light and rarely broken active matrix display device. The polyimide specifically used herein is prepared from 1,2,4,5-cyclohexanetetracarboxylic dianhydride as a tetracarboxylic acid component and 4,4′-diaminodiphenyl ether as a diamine component.
Patent Document 2 discloses a method for manufacturing a colorless transparent resin film formed of a polyimide excellent in colorless, transparency, heat resistance and planarity and used as a transparent substrate, a thin-film transistor substrate and a flexible wire substrate for liquid crystal display devices and organic EL display devices, by a solvent casting method using a particular drying step. The polyimide used herein is prepared from 1,2,4,5-cyclohexanetetracarboxylic dianhydride as a tetracarboxylic acid component and α,α-bis(4-aminophenyl)-1,4-diisopropylbenzene and 4,4′-bis(4-aminophenoxy)biphenyl as diamine components.
Patent Documents 3 and 4 disclose a polyimide soluble in an organic solvent, in which dicyclohexyltetracarboxylic acid is used as a tetracarboxylic acid component and diaminodiphenyl ether, diaminodiphenyl methane, 1,4-bis(4-aminophenoxy)benzene, 1,3-bis(4-aminophenoxy)benzene, 2,2-bis[4-(4-aminophenoxy)phenyl]propane, bis[4-(4-aminophenoxy)phenyl]sulfone, bis[4-(4-aminophenoxy)phenyl]ether or metaphenylenediamine is used as a diamine component.
Such a semi-alicyclic polyimide, in which an alicyclic tetracarboxylic dianhydride is used as a tetracarboxylic acid component and an aromatic diamine is used as a diamine component, has transparency, bending resistance and high heat resistance in combination. However, such a semi-alicyclic polyimide generally has a coefficient of linear thermal expansion as high as 50 ppm/K or more, which greatly differs from a coefficient of linear thermal expansion of a conductive material such as a metal, with the result that a failure such as large warpage may probably occur in a process of forming a circuitboard. In particular, there has been a problem that a process for forming a fine circuit for use in displays is not easily made.
In addition, such a semi-alicyclic polyimide tends to have insufficient solvent resistance and a failure might occur in a circuit forming process. Furthermore, this polyimide was sometimes inferior in flame resistance, decreasing the safety of electronic machines.